The usual method of quantitatively determining water is the Karl Fischer method, in which the substance under investigation is reacted with a solution of sulfur dioxide and iodine in a mixture of pyridine and methanol; see K. Fischer, Angew, Chemie, Vol. 48 (1935), p. 394. With water the reagent is converted into pyridine sulfate and hydrogen iodide with accompanying loss of color. The amount of iodine consumed is a measure of the water content of the substance. The reaction follows the equation: EQU SO.sub.2 +I.sub.2 +2H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +2HI
Determination by titration is a very precise method. With the aid of the reagent it is even possible to detect a water content of less than 0.01%; see Kirk and Othmer, Encyclopedia of Chemical Technology, 2nd edition, Vol. 2 (1963), pp. 673-677.
The Karl Fischer method has the drawback that the reaction is very slow, which means that titration is laborious and tedious and the end-point drags. The odor of sulfur dioxide and pyridine is disagreeable, so that it is even necessary to work under a hood. Another disadvantage is that the sulfur dioxide and iodine form the yellow SO.sub.2 I.sup.- complex, which makes it impossible to detect a visual end-point.
The limited shelf life, the instability of the titre, and the need for storage in a cool, dark place are further disadvantages.
Limited applicability and the inconsistency of the titration medium are further indicative of the problems faced by the analyst, despite the considerable improvements brought by the Karl Fischer method.
In a known development of this Karl Fischer method the problems of titrimetric determination of water are avoided; see J. C. Verhoef and E. Barendrecht, Analytica Chimica Acta, Vol. 94 (1977), pp. 395-403. This improved method makes use of two reagents, namely a solution of sodium acetate and sulfur dioxide in methanol (solution A) and a solution of iodine in methanol (titration solution B). In solution A, for example, the molarity for sodium acetate is 0.5 and the molarity for sulfur dioxide also 0.5. The solution has an APHA color index of 10 and the control value amounts to 0 to 4 ml of titration solution B for 20 ml of solution A. Titration solution B has a constant titre of 3.5 mg of H.sub.2 O/ml. Approximately 1 part titration solution B is required per 2 parts solution A.
The titration procedure is as follows: 20 ml of solution A is pre-titrated with titration solution B under continual stirring and in a moisture-free atmosphere. A specified amount of the hydrous substance under investigation is then quickly introduced into the titration vessel. The amount of the substance to be investigated (sample) should be in correct proportion to the estimated amount of water present. Taking into account the buffering capacity it is possible to determine 50 to 60 mg of water in 20 ml of solution A. The titration vessel is sealed, the burette adjusted and titration begun. The solution should be thoroughly mixed with a magnetic stirrer throughout the whole titration procedure.
The bipotentiometric method is used in the most conventional titration procedures to determine the end-point. Here reduction time is normally fixed at the point of equivalence at 20 seconds. The occurrence of yellow discoloration before the end-point is an indication for insufficient buffering capacity. This can be corrected by decreasing the sample amount or increasing the quantity of solution A. Using this method it is possible to smoothly carry out determination of water in alcohol, alkanes, aromatic hydrocarbons, aldehydes, ketones, ethers, esters, salts with water of crystallization, basic substances such as trishydroxymethylamino methane, lyophilized products, foodstuffs, molecular sieves and granular fertilizers. Visual end-point detection is also possible with this method.
This method has the disadvantage that when solution A is allowed to stand a milky-white murkiness or precipitate immediately forms, which is particularly adverse for the visual end-point detection. Moreover, the control value of solution A increases to an undesirably high degree as the solution ages and at rather high temperatures.
The problem underlying the invention is thus to develop a reagent for the quantitative determination of water consisting of solution A described above, used in combination with titration solution B, whereby the reagent does not form precipitates when left to stand and where the increase in control value is kept to a minimum.